Water refining in the mining industry has become the University of Cape Town’s (UCT’s) crystallisation and precipitation unit’s new focus of research, with a new technology called eutectic freeze crystallisation (EFC) showing promise in treating mine wastewater.
The unit was established in 2000 by Professor Alison Lewis, with the main focus on precipitation and crystallisation research, primarily related to the minerals processing industry.
Lewis notes that specific crystallisation projects in hydrometallurgy involve the precipitation of platinum-group metals, selenium, tellurium, nickel, manganese, aluminium and magnesium as elemental species or ionic salts.
UCT crystallisation and precipitation PhD researcher Dyllon Randall notes that the fact that South Africa is a water-scarce country, together with the group’s interest in hydrometallurgy processes, has resulted in an increased interest in water treatment. EFC was identified as a possible water treatment method to tackle the country’s mine water issue.
The research group currently has two PhD students and two MSc students working on the use of EFC as a means of treating mine water. The EFC project, which started in 2007, has no definite completion date, as it is a new area of research with a number of avenues to pursue. Randall tells Mining Weekly that the current goal is to build a pilot plant, followed by an EFC plant in South Africa.
The project involves the treatment of mine water using this unique technology, whereby the mine water is cooled to a specific temperature, forming both ice and salt. This temperature, explains Randall, is known as the eutectic tem- perature and is the point where both salt and ice will begin to crystallise and, because salt is denser than ice, it will sink to the bottom of the crystalliser, while the ice will float to the top.
Further, as each salt has its own eutectic temperature, a number of pure salts can be removed from a waste stream along with potable water. The volume of the waste stream can be further reduced by removing more salts along with more ice. The result, in theory, would be a 0% discharge. Randall notes that the technology was first initiated at the Delft University of Technology, in the Netherlands. However, the technology has never been applied to complicated systems such as mine water.
“The benefits of the project to the mining industry would be a near 0% waste stream from the treatment of the mine water as well as saleable salt products along with potable water,” says Randall.
The project is being funded by research associations Coaltech, the Water Research Commission, the Technology and Human Resources for Industry programme, UCT, as well as State-owned power utility Eskom, petrochemicals group Sasol, pla- tinum producer Impala Platinum and coal-mining major Anglo Coal.
Randall reports that the preliminary results of using EFC for the treatment of specific mine water show that pure calcium sulphate, pure sodium sulphate and potable water can be produced. The mass reduction amounts to ~97% of the initial feed, showing that the sequential removal of salts and the production of pure water are possible from mine water. He adds that, although mine water is generally a multicomponent aqueous stream, it can produce pure salts along with pure water and that the generated waste stream can be significantly reduced and, in theory, a 0% discharge can be achieved by removing all possible salts.
The Crystallisation and Precipitation Unit has achieved a number of milestones with this project, including the successful design and operation of a batch EFC setup for the sequential removal of salts; the successful commissioning of the Ice Laboratory for subzero temperature expe- riments; the two best paper awards at the 2009 international mine water conference; and the 2009 Water in Mining conference held in Perth, Australia.